At the time of our founding I doubt whether anyone would have wanted to outlaw muskets, even though, say a cousin of yours might have shot himself in the eye by being careless with a ram-rod.
Understanding the dangers inherent in loading a musket then is no different from understanding the risks or dangers of having sex with a potential aids carrier.
That a twelve-year-old might be able to safely handle a musket would not necessarily equate to being as proficient when considering sexual practices or partners. Therefore, given the varying gradations of risk involved with many activities I would favor restrictions on minors as well as prosecuting adults who'd act to undermine such restrictions.